As image processing after imaging, various images may be generated by separating diffuse reflection components from specular reflection components in an input image. For example, an image in which gloss is controlled may be generated by using diffuse reflection components and specular reflection components serving as gloss components which are separated from each other.
Furthermore, a surface normal required for generation of an image while lighting to an object is changed may be obtained with high accuracy. A view of an object is determined in accordance with information on a shape of the object, information on reflectance of the object, information on a light source, and the like. A physical behavior of reflection light obtained by reflecting light emitted from a light source by an object depends on a local surface normal, and therefore, in particular, a surface normal of the object is effectively used as the shape information instead of a 3D shape. As a method for obtaining a surface normal of an object, a photometric stereo method is used. In the photometric stereo method, a surface normal is obtained on the assumption that an object performs Lambertian diffuse reflection, and therefore, only diffuse reflection components of an input image are required. Therefore, a technique of estimating diffuse reflection components in an image is required. If diffuse reflection components in an image may be estimated, specular reflection components may be simultaneously obtained by subtracting the diffuse reflection components from the image.
PTL 1 and NPL 1 disclose a method for estimating diffusion reflection components in an input image based on pixels extracted for individual hues of the input image utilizing a fact that an object obeys a dichromatic reflection model. In the dichromatic reflection model, reflection light from an object may be represented by a linear sum of diffuse reflection components serving as an object color and specular reflection components serving as a light source color.
In this method, pixels are extracted for individual hues of an input image so that diffuse reflection components are estimated for individual objects having the same diffuse reflectance. When the light source emits white light, a hue of an object depends on only the diffuse reflection components serving as an object color and does not relates to presence or absence of the specular reflection components serving as a light source color, and therefore, pixels may be extracted only from objects having the same diffuse reflectance using hue.